Injection molding of plasticized polymeric materials including plastic and/or elastomeric type materials has been known and practiced for a long time. These type of apparatus are conventionally associated with various molding machines which are adapted for receiving the plasticized polymer and forming it into many useful parts and/or products.
Injection molding apparatus of the type alluded to are generally comprised of a feedscrew or auger member rotatably carried within a substantially cylindrical barrel, the screw effectively moving and plasticizing the polymeric material throughout the length of the barrel towards an exit end where it is forcefully injected into a molding machine for forming and curing of the material into the desired end product. At an exit end of the feedscrew there is conventionally mounted a check valve assembly which is designed to meter out the proper amount of plasticized material by a pressure reactive motion of the valve to effectively shut off the supply of material and to thereafter force the desired volume of plasticized polymer out of an exit nozzle and into the mold cavity of the molding machine.
Many improvements in this art have been suggested and applied to the injection molding apparatus and these, to an improved configuration of the feedscrew member and/or to the check valve assembly to gain greater effeciency in the injection operation by shortening the injection cycle period. Obviously, a shorter cycle period of the injection molding apparatus will also result in an increase in the number of parts which may be produced inasmuch as the molding machines associated therewith may also be configured to accept and form more individual parts.
These improvements in the injection molding apparatus have fairly coincided with advances in the polymer science and technology which has provided improved material chemistry. The improvements, however, have not been without problems as there is now a noticable increase in wear of the various member elements which comprise the injection molding apparatus. For example, it has been determined that no natural lubricants are available in many of the polymeric materials and this lack of lubrication increases the friction and therefore also the heat generated in plasticizing and injecting the material. Furthermore, it is not possible to add a lubricating material to the process as these tend to contaminate the polymer and this affects the quality of the finished molded product.
In view of the above, friction wear of critical working elements of the injection molding apparatus is a major and continuing problem in the industry as these must be replaced at regular and, in some instances, very short intervals. It is, of course, generally well-known and recognized by those knowledgeable in this art that the various working elements of the injection molding apparatus are comprised of very expensive tool and/or alloy steels, and this, because of the exceptional wear that these elements experience in this type of process. Thus, the very short service lifetimes of these elements will naturally also effect an increase in the cost of the molded parts being produced.
The following prior art patents fairly represent what has been done in attempts to improve the injection molding apparatus: U.S. Pat. Nos. 3,698,694; 4,106,113; 4,105,147; 4,472,058; and 4,988,281. Further U.S. Pat. No. 3,209,408 addresses the friction wear problem by providing a ball-bearing configured check valve assembly. Such type ball-bearing configurations are also evident in some of the above-listed prior art patents. In addition, U.S. Pat. No. 4,530,605 attempts to alleviate part of this problem by providing a rapid takedown configuration for a check valve assembly such that when worn parts need to be replaced this may be done quickly and effeciently with the least amount of down-time. From this it should be apparent that the friction wear problem of critical elements of an injection molding apparatus still exists and this, irrespective of the various advances in the art.
It is, therefore, in accordance with a primary aspect of the present invention an object to provide an improved check valve assembly for an injection molding apparatus wherein the service lifetimes of the various working elements is increased such that many more molded products may be produced before it becomes necessary to replace the working elements of the apparatus.
In accordance with another aspect of the invention it is an object to provide an improved injection molding apparatus check valve assembly which may be made from less expensive base metal and/or tool steel than now applied for these type elements while also providing an operational service life which is greatly extended over what is available with presently made check valve assemblies.